Low nickel containing chromim-nickel-mananese-copper austenitic stainless steel

ABSTRACT

An austenitic stainless steel includes (a) 0.03 wt % to 0.12 wt % of C, (b) 0.2 wt % to 1.0 wt % of Si, (c) 7.5 wt % to 10.5 wt % of Mn, (d) 14.0 wt % to 16.0 wt % of Cr, (e) 4.05 wt % to 4.31 wt % of Ni, (f) 0.04 wt % to 0.07 wt % of N, (g) 1.0 wt % to 3.5 wt % of Cu, (h) trace amount of Mo, and the balance being Fe and incidental impurities. The austenitic stainless steel has a δ-ferrite content less than 8.5 and equal to 6.77[(d)+(h)+1.5(b)]−4.85[(e)+30(a)+30(f)+0.5(c)+0.3(g)]−52.75.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part (CIP) of U.S. patent application Ser. No. 10/353,167, filed by the applicant on Jan. 28, 2003, and abandoned as of the filing date of this application. The entire disclosure of the parent application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an austenitic stainless steel, more particularly to a low nickel containing chromium-nickel-manganese-copper austenitic stainless steel.

2. Description of the Related Art

U.S. Pat. No. 5,286,310 discloses a low nickel containing chromium-nickel-manganese-copper austenitic stainless steel that has a reduced nickel content and acceptable metallographic structure, mechanical strength, corrosion resistance and workability. The aforesaid austenitic stainless steel contains at least 16.5% by weight of chromium so as to provide acceptable corrosion resistance. However, the chromium content should not exceed 17.5% by weight so as to prevent undesired formation of delta ferrite (δ-ferrite) during hot working and impairment to hot workability. The aforesaid austenitic stainless steel further contains at least 2.5% by weight of nickel so as to improve cold workability and so as to inhibit transformation of austenite into martensite. However, nickel content should not exceed 5% by weight due to the relatively high price thereof.

Although the aforesaid austenitic stainless steel is capable of providing acceptable corrosion resistance and cold or hot workability, the chromium content thereof is still high (previous investigation has shown that at least 17% by weight of chromium is necessary to provide minimum levels of corrosion resistance), which can impair stability of the austenitic stainless steel and which can cause cracking during hot rolling.

The disclosure of U.S. Pat. No. 5,286,310 is incorporated herein by reference.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a low nickel containing chromium-nickel-manganese-copper austenitic stainless steel that is capable of overcoming the aforesaid drawbacks of the prior art.

According to this invention, there is provided an austenitic stainless steel that comprises: (a) 0.03 wt % to 0.12 wt % of C; (b) 0.2 wt % to 1.0 wt % of Si; (c) 7.5 wt % to 10.5 wt % of Mn; (d) 14.0 wt % to 16.0 wt % of Cr; (e) 4.05 wt % to 4.31 wt % of Ni; (f) 0.04 wt % to 0.07 wt % of N; (g) 1.0 wt % to 3.5 wt % of Cu; (h) trace amount of Mo; and the balance being Fe and incidental impurities. The austenitic stainless steel has a δ-ferrite content that is less than 8.5 and that satisfies the following formula δ-ferrite=6.77 [(d)+(h)+1.5(b)]−4.85[(e)+30(a)+30(f)+0.5(c)+0.3(g)]−52.75.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawing which illustrates an embodiment of the invention,

FIG. 1 is a diagram illustrating the relationship between δ-ferrite content of the preferred embodiment of the austenitic stainless steel of this invention and hot working temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the low nickel containing chromium-nickel-manganese-copper austenitic stainless steel of the present invention comprises: (a) 0.03 wt % to 0.12 wt % of C; (b) 0.2 wt % to 1.0 wt % of Si; (c) 7.5 wt % to 10.5 wt % of Mn; (d) 14.0 wt % to 16.0 wt % of Cr; (e) 4.05 wt % to 4.31 wt % of Ni; (f) 0.04 wt % to 0.07 wt % of N; (g) 1.0 wt % to 3.5 wt % of Cu; (h) trace amount of Mo; and the balance being Fe and incidental impurities. The austenitic stainless steel has a δ-ferrite content that is less than 8.5 and that satisfies the following formula δ-ferrite=6.77[(d)+(h)+1.5(b)]−4.85[(e)+30(a)+30(f)+0.5(c)+0.3(g)]−52.75, wherein (a), (b), (c), (d), (e), (f), (g), (h) in the formula mean the content of the respective elements (wt %).

The austenitic stainless steel can further comprise 5 to 30 ppm of B so as to improve hot workability. The contents of harmful impurities, such as S (sulfur) and P (phosphorous), are as small as possible. However, due to cost concerns associated with removal of these impurities, the S content is limited to 150 ppm, and the P content is limited to 0.06 wt %.

FIG. 1 illustrates the relationship between the δ-ferrite content of the preferred embodiment of the austenitic stainless steel of this invention and temperature. The results show that when temperature is raised to above 1250° C. during hot rolling, the δ-ferrite content rises sharply, which results in the risk of edge cracking of a rolled plate of the austenitic stainless steel. In addition, a minimum temperature of 1050° C. during hot rolling is required so as to obtain the requisite mechanical strength.

EXAMPLES AND COMPARATIVE EXAMPLES

The following Examples and Comparative Examples illustrate the unexpectedly better results of this invention over the prior art.

Table 1 illustrates an edge crack effect test for different test specimens of the austenitic stainless steel of Examples 1 to 9 and comparative Examples 1 to 5, which differ in composition (only elements Ni, C, Si, Mn, Cr, Cu, and N are shown). The test was conducted by hot rolling at a temperature ranging from 1050° C. to 1250° C. The test results show that each Example of the austenitic stainless steel of this invention has a δ-ferrite content less than 8.5, and that no edge cracking was observed for the test specimens of Examples 1 to 9. Each of the test specimens of the Comparative Examples 1 to 5 has a δ-ferrite content greater than 8.5. Edge cracks were found in each of the test specimens of the Comparative Examples 1 to 5. The results shown in Table 1 demonstrate that edge cracks can be avoided when the Ni content ranges from 4.05% to 4.55% with the Nitrogen content ranging from 0.04% to 0.061%. TABLE 1 Ni C Si Mn Cr Cu N δ-ferrite Edge crack Examples 1 4.31 0.053 0.50 7.60 16.30 1.60 0.041 8.49 None 2 4.05 0.032 0.53 7.85 15.36 1.71 0.04 6.636 None 3 4.07 0.032 0.54 8.00 15.33 1.66 0.043 6.259 Noen 4 4.55 0.032 0.58 7.54 15.23 1.59 0.041 4.984 None 5 4.15 0.059 0.62 7.44 15.26 1.65 0.042 3.859 None 6 4.24 0.046 0.42 7.86 15.68 1.66 0.061 3.278 None 7 4.21 0.051 0.49 7.63 15.16 1.62 0.041 1.684 None 8 4.09 0.060 0.50 8.08 15.14 1.70 0.042 0.109 None 9 4.19 0.066 0.54 7.76 14.99 1.65 0.044 −1.989 None Comparative Examples 1 4.31 0.039 0.47 7.07 19.04 2.15 0.039 28.58 Cracking 2 4.36 0.05 0.45 7.58 17.53 2.03 0.039 15.82 Cracking 3 4.37 0.046 0.47 7.96 18.33 1.71 0.035 22.60 Cracking 4 4.77 0.052 0.51 7.54 18.13 1.73 0.032 19.85 Cracking 5 4.45 0.051 0.53 7.5 16.20 1.5 0.031 9.1 cracking

Table 2 illustrates a corrosion resistance test (ASTM B117) using salt fog for different test specimens of the austenitic stainless steel of Examples 10 to 12 and comparative Example 6 (type 304 stainless steel), which differ in composition (only elements Ni, C, Si, Mn, Cr, Cu, and B are shown) The test results show that each Example of the austenitic stainless steel of this invention has a corrosion rate that is as low as that of the type 304 stainless steel (no more than 0.1%) of the prior art. TABLE 2 Corro- sion Ni C Si Mn Cr Cu B rate Exam- ples 10 4.40 0.058 0.48 7.56 15.26 1.79 0.0001 ≦0.1 wt % 11 4.11 0.051 0.54 7.86 15.35 1.69 0.0032 ≦0.1 wt % 12 3.40 0.059 0.77 7.84 14.94 1.78 0.0001 ≦0.1 wt % Compar- ative Exam- ple  6 8.02 0.045 0.53 1.25 18.19 0.23 0.0008 ≦0.1 wt %

It is noted that the chromium content in each of the Examples 1 to 12 of the austenitic stainless steel of this invention is less than 17 wt %, which is a minimum requirement of the prior art for providing minimum levels of corrosion resistance.

Table 3 illustrates compositions of test specimens of the austenitic stainless steel of Examples 13 to 22 and comparative Examples 7 to 10 (only elements Ni, C, Si, Mn, Cr, and Cu are shown). Table 4 illustrates a mechanical strength test for the test specimens of the austenitic stainless steel of the Examples 13 to 22 and the comparative Examples 7 to 10. The test results show that the austenitic stainless steel of this invention has an elongation better than those of type 304 stainless steel of the prior art. Other mechanical properties, such as tensile strength, yield strength, and hardness, of the austenitic stainless steel of this invention are comparable to those of type 304 stainless steel of the prior art. TABLE 3 Ni C Si Mn Cr Cu Examples 13 4.26 0.036 0.56 7.7 15.12 1.67 14 4.21 0.039 0.47 7.97 15.32 1.66 15 4.21 0.056 0.54 7.69 15.26 1.79 16 4.15 0.049 0.48 7.7 15.26 1.66 17 4.20 0.040 0.49 7.93 15.35 1.67 18 4.21 0.039 0.48 7.96 15.29 1.66 19 4.22 0.044 0.46 7.93 15.01 1.70 20 4.17 0.064 0.5 7.71 15.16 1.65 21 4.20 0.055 0.52 7.70 15.32 1.68 22 4.41 0.058 0.48 7.56 15.27 1.80 Comparative Example  7 8.06 0.039 0.53 1.17 18.14 0.23  8 8.04 0.041 0.50 1.15 18.15 0.21  9 8.08 0.039 0.49 1.18 18.17 0.24 10 8.03 0.040 0.52 1.11 18.09 0.22

TABLE 4 Tensile strength, Yield strength, Hardness, Elongation, (MPa) (MPa) (HRBO) (%) Examples 13 621.7 313.3 83.5 55.2 14 630.2 289.5 82.5 55.3 15 628.5 287.6 82.3 55.0 16 642.3 291.3 82.8 53.1 17 618.4 312.0 84.3 53.7 18 634.6 296.4 82.8 53.8 19 639.0 317.2 83.9 54.1 20 642.6 319.7 84.7 54.3 21 621.7 313.3 83.5 55.2 22 641.9 301.6 83.4 53.4 Comparative Examples  7 660.0 324.6 83.2 49.1  8 660.6 325.0 82.6 46.8  9 663.8 328.9 82.4 48.8 10 657.8 322.8 81.8 48.5

The aforesaid tests show that the austenitic stainless steel of this invention is capable of exhibiting excellent mechanical strength, corrosion resistance, and phase stability during hot or cold working with a relatively low nickel content and a low chromium content as compared to those of the prior art.

With the invention thus explained, it is apparent that various modifications and variations can be made without departing from the spirit of the present invention. 

1. An austenitic stainless steel comprising: (a) 0.03 wt % to 0.12 wt % of C; (b) 0.2 wt % to 1.0 wt % of Si; (c) 7.5 wt % to 10.5 wt % of Mn; (d) 14.0 wt % to 16.0 wt % of Cr; (e) 4.05 wt % to 4.31 wt % of Ni; (f) 0.04 wt % to 0.07 wt % of N; (g) 1.0 wt % to 3.5 wt % of Cu; (h) trace amount of Mo; and the balance being Fe and incidental impurities; wherein said austenitic stainless steel has a δ-ferrite content that is less than 8.5 and that satisfies the following formula δ-ferrite=6.77[(d)+(h)+1.5(b)]−4.85[(e)+30(a)+30(f)+0.5(c)+0.3(g)]−52.75.
 2. The austenitic stainless steel of claim 1, further comprising 5 to 30 ppm of B.
 3. The austenitic stainless steel of claim 1, further comprising no more than 150 ppm of S.
 4. The austenitic stainless steel of claim 1, further comprising no more than 0.06 wt % of P. 